Gaseous fuels, such as natural gas, are promising candidates for fueling diesel engines because of their ready availability and potential for reducing particulate emissions. When gaseous fuel is injected directly into an engine's combustion chamber at the end of the compression stroke, in a so-called "direct injection" engine, a further benefit is that the high efficiency characteristics of diesel engines is maintained. To overcome the cylinder pressure near the end of the compression stroke (near top dead center), a higher gas injection pressure is required for the gaseous fuel to enter the combustion chamber. When the injection valve assembly employs a second fluid, leakage of the gaseous fuel into the compartments of the injection valve containing the second fluid can adversely affect the operation of the injection valve. For example, if the gaseous fuel injection valve is hydraulically actuated, leakage of the gaseous fuel into the actuating fluid could contaminate the hydraulic actuation circuit so as to prevent or inhibit actuation. Known, conventional hydraulically actuated liquid fuel injection valves have traditionally relied on extremely low diametric clearances between the bore and the reciprocating valve needle disposed within the bore to reduce leakage of pressurized liquid fuel into the hydraulic actuation fluid and vice versa. This method, however, does not provide a positive seal between the liquid fuel and hydraulic actuation fluid and therefore does not substantially eliminate leakage but simply reduces it. A gaseous fuel has extremely low viscosity and thus low diametric clearances are ineffective for sealing low viscosity fluids. Accordingly, low diametric clearances are, in general, insufficiently reliable for providing effective sealing for hydraulically actuated gaseous fuel injection valves.
Known, conventional sealing strategies for gaseous fuel injection valves have traditionally involved )-rings or other soft or polymeric material seals, which act to prevent leakage of pressurized fuel into the other injection valve compartments. However, a drawback of using traditional elastomeric O-rings in high-pressure direct injection systems is the general inability of conventional O-ring materials to withstand rapid reciprocation rates and the high-pressure conditions found within a fuel injection valve without a severe reduction in operative lifespan. In response to the foregoing limitations, sealing techniques employed in gaseous fuel injection valves have evolved so as to incorporate fluid seals.
Fluid seals in gaseous fuel injection valves typically employ a pressurized sealing-fluid that prevents leakage of gaseous fuel into other compartments within the injection valve and/or into a second fluid, such as hydraulic fluid or a secondary fuel. Provided that the pressure of the sealing-fluid is greater than that of the gaseous fuel, the gaseous fuel will not leak past the sealing-fluid and into the second fluid. Preferably, the sealing-fluid is also combustible, such that a small amount of leakage of sealing-fluid into the fuel is acceptable.
A fluid seal is described in U.S. Pat. No. 5,163,397 (the '397 patent), issued Nov. 17, 1992. The '397 patent describes a pilot fuel injection pump that comprises plunger that reciprocates in a bore. A sealing-fluid is pressurized in an annular groove provided in the surface of the plunger to form a fluid seal. The purpose of the seal is to prevent leakage of the pilot fuel past the fluid seal.
A further example, U.S. Pat. No. 5,890,459 (the '459 patent), issued Apr. 6, 1999, discloses an injection system for a dual fuel direct injection combustion engine. The disclosed system comprises three separate injection valves for introducing fuel into a combustion chamber. A liquid seal employs a pump that pressurizes a sealing-fluid to a constant pressure that is higher than that of the combustible gaseous mixture.
For systems designed to handle gaseous fuels, it is important that the sealing-fluid be maintained at a pressure level higher than the gaseous fuel pressure; otherwise the gaseous fuel may breach the fluid seal and leak out, resulting in inefficiencies caused by lost fuel and possibly operational difficulties, if for example, the gaseous fuel leaks into a hydraulic fluid. Fluid seal systems, such as those disclosed in the '397 patent and the '459 patent, typically maintain the sealing-fluid pressure at a constant level which is higher than the highest anticipated gaseous fuel pressure.
U.S. Pat. No. 5,996,558 (the '558 patent), issued Dec. 7, 1999, which is co-owned along with the present application by Westport Research Inc., discloses a hydraulically actuated gaseous fuel injection system in which the gaseous fuel pressure may vary as a function of engine speed and other engine load conditions in order to improve combustion. U.S. Pat. No. 5,771,857 (the '857 Patent), issued Jun. 30, 1998, also discloses a variable fuel gas pressure control system for a direct injected internal combustion engine where the fuel gas pressure varies according to engine load. Accordingly, it is desirable to provide a fluid seal that is dynamically maintained at a pressure greater than the changing pressure of the gaseous fuel. Maintaining the sealing-fluid pressure at a constant level that is higher than the anticipated maximum gaseous fuel pressure, results in an excessive amount of sealing-fluid leaking into the gaseous fuel when the gaseous fuel pressure is much lower than the predetermined maximum gaseous fuel pressure, since, during these times, the pressure differential between the sealing-fluid and the gaseous fuel is excessively high. When the sealing-fluid pressure is higher than the gaseous fuel pressure, some of the sealing-fluid flows in a laminar or restricted fashion through the diametric clearance gap between the bore and the reciprocating valve needle disposed within the bore. The restricted flow of sealing-fluid can be described by the following equation: EQU Q.sub.SD =(kc.sup.3.DELTA.P).div.L,
where Q.sub.SD is the flow of the sealing-fluid, k is a constant, c is the diametric clearance between the valve and the valve chamber, .DELTA.P is the pressure difference between the sealing-fluid in the annular groove and the gas in the gas chamber, and L is the length of the bore between the annular groove and the valve fuel chamber within the injection valve. Machining capabilities limit reductions in diametric clearance c and space constraints typically limit increases in length L. However, by reducing .DELTA.P, leakage of sealing-fluid into the valve fuel chamber may be reduced. Accordingly, as .DELTA.P increases, the flow of sealing-fluid into the valve fuel chamber increases, resulting in the undesirable consumption and combustion of excessive amounts of sealing-fluid. Inefficient and potentially damaging combustion of sealing-fluid is exacerbated in fuel cut-off conditions, where the supply of gaseous fuel to the combustion chamber is arrested. An example of a fuel cut-off condition is when the vehicle is going down a steep hill and engine compression is being used to slow the vehicle. In such a situation, the engine speed is adequate without additional combustion. During fuel cut-off conditions, a significant amount of sealing-fluid may accumulate in the gaseous fuel chamber within the injection valve, and when fuel injection recommences, the accumulated sealing-fluid will be injected into the engine combustion chamber and combusted in the first engine cycle with undesirable environmental and potential equipment-damaging side effects. Leakage of the sealing-fluid in the above-described manner may also result in over-fueling. If the level of fuel in the combustion chamber becomes too excessive, when combustion recommences after a fuel cut-off condition, engine components such as the pistons, the cylinder head, connecting rods and the crankshaft may be over-stressed.
The present sealing apparatus and method overcome the problems set forth above by reducing the pressure differential between the sealing-fluid and the gaseous fuel and by linking the pressure of the sealing-fluid and the gaseous fuel so that the pressure of one fluid is used to dynamically control the pressure of the other fluid. That is, the pressure of the sealing-fluid can change dynamically in response to the changes in the pressure of the gaseous fuel. The present apparatus and method thus provide an improved system over conventional systems that employ a constant pressure sealing-fluid.
It is further desirable to have a sealing system that overcomes the traditional problems of durability under extreme reciprocation rates and pressure conditions that have limited the use of O-ring type seals in fuel injection valves.
It is further desirable to have a fluid sealing system that overcomes the efficiency, environmental and engine component integrity problems associated with excessive leakage of sealing-fluid into the fuel during "fuel cut-off" conditions.